1. Field of the Invention
The present invention relates to filament type devices for trimming vegetation.
More particularly, the present invention relates to filament type trimming devices wherein a cutting filament is fed out from a spool and is supported in a plane radial to the shaft axis for rotary cutting movement therein.
2. Description of the Prior Art
Filament type vegetation trimmers in which a filament is rapidly slung radially about an axis to provide a cutting element are well known in the art as a particularly effective alternative to bladed trimmers.
The filament used generally comprises a plastic material and is necessarily more fragile than blades and susceptible to wear and breakage. Accordingly, filament trimmers known in the art generally utilize individual, replaceable filaments or carry a continuous supply of filament which may be fed out gradually and/or incrementally as the exposed cutting filament is worn away. Filament trimmers of the latter type may carry their filament supply on the rotating slinger head or the supply may be carried on a fixed member such as the handle, and fed out to the slinger head for rotation therewith.
Obviously, a fixed filament supply is advantageous in that power is not wasted in effecting unproductive rotation thereof. However, guiding a portion of a filament extending through a longitudinal feed passage into a generally horizontal orientation for cutting in a radial plane has presented significant problems.
For example, U.S. Pat. No. 3,928,911 teaches an open, bell mouthed slinger head in direct, open communication with an axial bore in the power output shaft which is provided with one or more radial grooves for engaging and swinging the filament about the shaft axis. However, initial engagement of the filament with the groove and maintenance of such engagement and swinging support during use have been found to be impracticable.
This problem can be obviated by providing a device wherein a filament is fed out through a bore in the power output shaft with a duct through the slinger head, which duct projects and, thereby, supportively guides filament in a direction generally radial from the output shaft. While this teaching would overcome many of the difficulties of the prior art, filament supported in such a duct would be particularly susceptible to wear and breakage as a result of heat generated by friction and centrifugal force exerted on the filament during use.
In such trimming devices wherein the filament is guided through a longitudinal feed passage and into a plane radial to the axis of the feed passage, the filament must extend through a bend radius interposed between its axially and radially extending portions. During operation, each time the filament is slung radially about the shaft axis, that is once each revolution, the filament twists about its axis. Accordingly, to prevent the filament from twisting up, once each revolution, the filament must rotate about its own axis to untwist and thereby to return to its original, axial condition. This untwisting is effected due to the resiliency inherent in the filament composition and is necessary each revolution as in its absence the filament line would twist tightly around itself. However, the twisting and untwisting together with the centrifugal force exerted on the cutting filament generates heat, particularly at the bend, the amount of heat being directly proportional to the acuteness of the bend radius and other factors such as speed of rotation and filament diameter.
When so heated, the plastic filament material is softened and is then unable to rotate about its axis to untwist in response to the radial movement. The continued rotation then causes the filament to twist up and, along with the softened condition of the plastic, to break. Thus, filament is wasted and the trimmer rendered less economical to operate. Moreover, as the heating, twisting and breaking of the filament occurs within a few minutes of operation, the convenience and desirability of this variety of trimmer are substantially diminished. Although lengthening the bend radius to render same less acute reduces the frictional heat generated therein and so, improves filament life, such lengthening necessarily increases the required length of the guide duct. Increase in the length of the guide duct necessitates increase in the size and resulting weight of the slinger head, with proportionate reduction in economy and ease of manufacture and use.
Electric powered trimmers in which filament is fed out through a bore in the armature shaft present particular difficulties in controlling filament softening. Most known trimmers comprise a closed filament storage and feed system and provide no airflow therethrough. Although it would be possible to achieve some degree of airflow by opening the filament storage and feed system to permit air to flow therethrough, to achieve sufficient airflow for effective filament cooling at its bend in the bend radius portion of the tube, it would be necessary to provide a significantly enlarged armature shaft bore. This requirement necessitates uses of a substantially larger and therefore heavier motor and imposes corresponding difficulties in manufacture and use.
The present invention obviates the problems of the prior art by providing means for cooling the filament during operation without corresponding difficulties or inconveniences in manufacture and use, and provides for a substantially increased filament life, thereby enhancing significantly the usefulness, convenience and desirability of this variety of filament type trimmer.